The present invention relates to an antenna circuit.
There are known diversity antenna circuits to which multiple antennas can be attached and over which a signal path can be connected to an output of a corresponding antenna circuit, with one antenna signal being transmittable on each signal path. If interference in the reception signal is detected with such a diversity antenna circuit, the signal is switched from one antenna to the next.
U.S. Pat. No. 5,697,075 describes a diversity antenna arrangement for a GPS (global positioning system) where an antenna having better reception sensitivity can be selected from among several antennas. At least two antennas are provided here. Comparison means are provided, which compare a direct component that has been amplified and output from the respective signal path of the reception antennas with a predetermined direct voltage. Then one of the two antennas is selected as a function of this comparison.
U.S. Pat. No. 5,559,838 also describes a diversity receiver including a plurality of receivers, each of which receives a plurality of signals and includes a detector that selects and outputs a desired signal from the received signals. A level comparator compares the signal levels of the desired signals supplied by the detectors. The desired signal having the highest signal level is determined in this way. This signal is then selected and demodulated.
The antenna circuit according to the present invention has the advantage over the related art that transmission of a second antenna signal over a second signal path is attenuated by a predetermined attenuation value in comparison with transmission of a first antenna signal over a first signal path. An analyzer unit is provided to determine the level of a weighting signal which is derived from an antenna signal transmitted to the output of the antenna circuit; an operating device is provided to connect only the first signal path to the output of the antenna circuit when the level determined is below a first predetermined level, and this operating device connects only the second signal path to the output of the antenna circuit when the level determined is above the first predetermined level. In this way, it is possible to improve reception of antenna signals in regions having very strong transmitters. Reception of antenna signals is often greatly impaired in regions having very strong transmitters. High reception levels lead to intermodulation in semiconductor components of an antenna amplifier and downstream receiver stages and thus interfere with the received antenna signals. However, as a result of switching between different signal paths to connect the output of the antenna circuit to the antenna connection device, the level at the output of the antenna circuit is limited to a level that does not result in the intermodulation described above.
It is especially advantageous that in the case when the second signal path is connected to the output of the antenna circuit, the operating device disconnects the second signal path from the output of the antenna circuit and connects the first signal path to the output of the antenna circuit when the analyzer unit detects a level below a second predetermined level, the second predetermined level being lower than the first predetermined level by more than the attenuation value. This results in a hysteresis for switching between the two signal paths, thus preventing excessively frequent switching.
It is advantageous that the operating device includes at least one, preferably mechanical, relay by way of which either the first signal path or the second signal path can be connected to the output of the antenna circuit. This does not result in any intermodulation in the received antenna signal.
Another advantage is obtained if the operating device includes at least one first switch and one second switch, the first signal path being connectable to the output of the antenna circuit over the first switch, and the second signal path being connectable to the output of the antenna circuit over the second switch, and the first switch or the second switch being designed as an electronic switch, preferably a PIN diode, and being arranged between the matching network and the output of the antenna circuit. This saves on costs and space for implementation of the operating device, especially in reception of HF antenna signals. Positioning the respective electronic switch between the matching network and the output of the antenna circuit guarantees good isolation between the signal paths to be switched, because each electronic switch is thus used at a point in the antenna circuit adapted to the impedance of the entire reception system.
Another advantage is obtained when an attenuation element is connected upstream from at least the first switch. This prevents excessively high levels from occurring at the respective switch, which can result in interfering intermodulation when the respective switch is designed as an electronic switch. The upstream attenuation element can be implemented in an especially simple manner by a multi-stage design of the respective switch, in particular in the form of two successive PIN diodes. At the first attenuation stage or PIN diode, interfering intermodulation products can be allowed to occur at a suitably high level of the received antenna signal, because they can then be switched off by the second switch stage or the downstream PIN diode. At the same time, the attenuated signal path is then also connected to the output of the antenna circuit.
An especially simple implementation of the antenna circuit is obtained when a second antenna can be connected to the antenna connection device, with the second antenna supplying an antenna voltage which is lower than that supplied by the first antenna by the predetermined attenuation value or having a lower reception sensitivity than the first antenna by the predetermined attenuation value, and the first antenna being connectable to the output of the antenna circuit over the first signal path and the second antenna being connectable to the output of the antenna circuit over the second signal path.
Antennas and matching networks can be eliminated by diverting the second signal path from the first signal path and by having the attenuation on the second signal path be higher than that on the first signal path by the predetermined attenuation value.
The second signal path can be diverted easily from the first signal path using a transformer or via a resistor network.
It is especially advantageous that the weighting signal corresponds to an antenna signal transmitted to the output of the antenna circuit. The weighting signal can be formed especially easily in this way.
It is advantageous if the weighting signal corresponds to a control signal which is derived from a received antenna signal to a radio receiver connected to the output of the antenna circuit, the level of the control signal increasing with intermodulation interference in the antenna signal. In this way, switching between the signal paths can take place only as a function of intermodulation occurring in the radio receiver, so that unnecessary switching is prevented.